Errors, errors & errors after installing WPDM in fresh vanilla WordPress 4.7.5

(possibly triple posting, got errors while editing my post)

I just installed a plain vanilla WordPress 4.7.5 instance on my VPS.
Configured minimal basic settings, WordPress works fine out of the box.
Installed WordPress Download Manager 2.9.51 straight after.
Installation went fine.
As soon as I entered a downloadable product, I got these errors in top of the screen:

Warning: Illegal string offset 'administrator' in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 521

Warning: Illegal string offset 'administrator' in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

Warning: A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

Warning: A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 523

Continued despite of errors, and added a new page named “Catalog”.
Placed the shortcode on the page that was listed in the download product line:

[wpdm_package id='4']

Saved the page and visited the site to view the result.
Resulted in the following errrors:

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522

<strong>Warning: </strong>A non-numeric value encountered in /apps/wordpress/public/wp-content/plugins/wpdm-premium-packages/includes/libs/functions.php on line 522
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This is my first introduction to WordPress Download Manager.
I have no idea what I did wrong or what I could have done to prevent this.
Any suggestions welcome.